Apparatus and method for bar code printing

ABSTRACT

There is disclosed an apparatus and method for controlling a row of ink jet streams to produce printed bars. Ink drops generated by a first group of streams are periodically gang charged and caught under the control of an input clock signal. Those of the drops which are uncharged deposit upon a moving print receiving medium to print a series of regularly spaced bars. A second group of streams within the same row are gang switched and caught under the joint control of the clock signal and a binary code signal. This latter group of streams prints extensions on selected ones of the bars printed by the first group of streams. System operating conditions are adjusted to produce overlapping of the printed dots which collectively define the bars.

[ 1 Jan. 22, 1974 United States Patent 91 Duffield May 1969; pp. 1736-1737 CODE PRINTING Inventor: Peter Leonard Duffield, Dayton, Primary EIami'1eTJ0$ePh Hartary APPARATUS AND METHOD FOR BAR Attorney, Agent, or Firm-Lawrence B. Biebel et al.

[73] Assignee: The Mead Corporation, Dayton,

Ohio

[57] ABSTRACT There is disclosed an apparatus and method for con- [22] Filed: Sept. 18, 1972 21 A L N trolling a row of ink jet streams to produce printed 1 PP 0 289,772 bars. Ink drops generated by a first group of streams are periodically gang charged and caught under the [52] U.S. CI. 346/75 control of an input clock signal. Those of the drops in! CI ld 15/18 which are uncharged deposit upon a moving print re- [58] Field of 3 6/75, 1 ceiving medium to print a series of regularly spaced bars. A second group of streams within the same row [56] References Cited UNITED STATES PATENTS are gang switched and caught under the joint control of the clock signal and a binary code signal. This latter group of streams prints extensions on selected ones of the bars printed by the first group of streams. System 3,560,641 2/1971 Taylor et al.

3,577,198 5/1971 Beam.......................

3,596,276 7/1971 Lovelady OTHER PUBLICATIONS Gamblin et a1; Electrostatic Ink Deflection Bar Code Printer; IBM Tech. Disc. Bulletin; Vol. 11, No. 12,

13 Claims, 5 Drawing Figures APPARATUS AND METHOD FOR BAR CODE PRINTING BACKGROUND OF THE INVENTION This invention relates to the field of bar code printing and more particularly to such printing by non impacting printing apparatus. Such apparatus are useful for printing bar codes on bulky or non-uniform items such as mail pieces and the like. After imprinting by a suitable printer, the bar code may be read by a compatible optical reader and used for controllingsorting operations or the like.

While a number of devices are available for non impact printing of bar codes, there has been a great deal of interest in using jet printers for this purpose. This is because ink jet printers are reasonably inexpensive, make dark printed marks, and can print on almost any type of paper. An example of one such prior art printer is shown in Lovelady et al. U.S. Pat. No. 3,596,276. Other types of ink jet printers have also been applied to the task of printing bar codes, but in general these printers are all alike in that they generate a stream of drops and thereafter scan the drop stream or the drop generator laterally back and forth for production of a printed bar. While such prior art printers may print reasonably good bar codes at fairly slow speeds, their printing quality has been observed to fall off when the printed piece moves at a higher velocity. In this regard a high speed may be something in the order of about 200 inches per second. At such speeds the prior art printers tend to become frequency response limited due to lateral scanning requirements. As a matter of fact, inertial limitations practically prevent the use of mechanically scanning printers at these speeds and, even low inertia jet scanning printers experience difficulty at the bar ends where the jet must decelerate to zero speed and accelerate to high speed in a reverse direction. A common consequence is scattered errant drops and hooked patterns at the bar ends.

SUMMARY OF THE INVENTION This invention provides apparatus and method for improved non-contact bar code printing at higher speeds than have previously been possible. This has been accomplished by producing a row of fluid jet streams, stimulating the streams to produce a curtain of uniformly sized and regularly spaced drops, and digitally switching selected drop groups in accordance with the changing state of a binary input control signal. Digital switching of drop groups is achieved by gang charging of simultaneously generated groups of drops, deflecting the drops in correspondence to their charges, and thereafter selectively catching drop groups in correspondence with the deflection thereof.

The fluid streams are produced by supplying a printing liquid under pressure to an orifice plate having a row of regularly spaced orifices therein. The printing fluid passes through these orifices and exits therefrom as a row of regularly spaced fluid filaments which in turn are stimulated as above mentioned to break up into a curtain of uniformly sized and regularly spaced drops. Ganged charging of simultaneously generated drop groups is accomplished by applying a charge control signal to one or more elongated electrodes situated near the point where the fluid filaments break up into drops. For printing a bar/half-bar code there are provided two such electrodes of equal length. A clocking signal and a binarily coded information signal jointly control the charging action of these electrodes to enable selective deflection by an electrostatic deflection field and selective catching by a common catcher. Overlap of the printed dots in the two principal coordinate directions is achieved by appropriate adjustment of the fluid supply pressure, the drop stimulation frequency and the speed of movement of the print receiv' ing medium.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a diagrammatic cross sectional view of apparatus made in accordance with this invention;

FIG. 2 is a pictorial representation of the charging, catching and drop deposition action of a bar/half-bar printer;

FIG. 3 is an enlarged representation of a long printed bar;

FIG. 4 is a pictorial representation of an assembly for a complete print head; and

FIG. 5 is a partially cut away view of an assembled print head as seen from underneath.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a bar code printer according to the practice of this invention there is generated a row of ink jets which are broken up into trains of uniformly sized and regularly spaced drops by apparatus as shown for instance in Beam et al U.S. Pat. No. 3,577,198. During drop formation most of the drops are charged for deflection and catching by a common catcher. However, selected groups of drops are permitted to remain uncharged. These drops avoid catching, and impact upon a print receiving member. Drop generation is adjusted such that there is overlapping of dots printed by simultaneously impacting drops from adjacent trains, and the print receiving member is moved at a speed such that there is overlapping of dots printed by successively impacting drops from the same train.

Thus there is provided an arrangement as shown in schematic cross section in FIG. 1 wherein a reservoir of ink 10 is maintained under pressure within a manifold 11. There is provided an orifice plate 12 having a series of orifices 13 through which the ink flows to form a series of filaments 14. A stimulator 15 is threaded into manifold l 1 and transmits regular frequency vibrations to the ink therein. This causes the filaments 14 to break up into trains of drops 16 for catching or deposition as shown in FIG. 2.

There are provided a pair of non-conductive charge strips 17 which have conductive areas 18 coated thereon. The coated areas 18 present elongated electrodes 30 on the inner edges of charge strips 17. As shown in FIG. 2, the charging surface of each of electrode areas 30 is of sufficient extent to provide a uniform electrical field for charging of five adjacent filaments 14. In charging the drops 16 generated by the five associated ink filaments 14, each charging electrode 30 cooperates with an opposing charging area on the other charge strip.

An electrostatic field for deflection of the charged drops is set up between a face portion of deflection electrode 19 and the conductive face 22 of catcher 21. Electrode 19 is a non conductive probe with a conductive area 20 coated thereon. The drops which are so charged impact the face of catcher 21 and run downwardly for injestion into an open slot directly above blade 23.

Operation of the apparatus as a bar code printer is accomplished by gang charging and catching of the drops as shown in FIG. 2. The apparatus is operative to print either long bars 24 or short bars 25 on a print receiving medium 26. As illustrated in FIG. 2, there are drop streams. Five of these streams are reserved for printing only the extended portions of the long bars 24. The other five streams print short bars and the non extended portions of long bars 24. These latter five streams are switched on and off at a regular frequency under the control of a clocking signal. The drops 16b which are deposited by these five streams create bar strokes at a frequency equal to the clocking frequency. The spacing between these bar strokes depends upon the clocking frequency and the speed of movement of receiving member 26. Print receiving medium 26 which may be a piece of mail, a continuous web of paper, or other recording medium moves in the directionof the arrow to create the regularly spaced bar strokes sequence illustrated in FIG. 2. This regularly spaced stroke pattern may be used to generate a clocking signal for an optical reader which will scan the bar code at some later time.

Drops 16a which print the extended portions of long bars 24, are switched under the joint control of the above-mentioned clocking input signal and a binary code signal. As an example of a convenient circuit for accomplishing this joint control, there may be provided a pair of input terminals 32 and 33, an AND gate 34, and a pair of inverting amplifiers 35 and '36. Input terminal 33 provides clock pulses directly to inverting amplifier 36 which provides charging signals to one of electrodes for charging of drops 16b. Amplifier 36 must invert the clock pulses because printing or drop deposition is accomplished by generation of uncharged drops.

The electrode 30 which controls the charging of drops 16a is driven by the output of inverting amplifier 35. Inverting amplifier in turn is connected to AND gate 34 which has inputs from both of terminals 32 and 33. A binary code signal comprising a series of short pulses is applied to the terminal 32. The code generating apparatus which creates the binary code will ordinarily generate code pulses only during clock pulse periods. Thus the presence of a code pulse during a clock period may correspond to a binary one while the absence of a code pulse during a clock period corresponds to a binary zero. This being the case, AND gate 34 may be functionally unnecessary, but it is shown for completeness of illustration. In any event, the joint occurance of a code pulse and a clock pulse produces a zero output from inverting amplifier 35 so that a long bar extension is printed. Simultaneously the presence of the clock pulse causes a zero output from inverting amplifier 36 so that drops 16b print'the non extended portions of the same long bar. Consequently each printed short bar 25 corresponds to the occurence of clocked zero at input terminal 32, and each long bar 24 corresponds to the occurence of a clocked one at input terminal 32. Thus there is printed a bar code of the type commonly known as a bar/half-bar code.

It will be apparent that other types of bar codes may be printed by the apparatus and method of this invention. For instance, a single charging electrode 30 could print a code comprising a series of bars of equal length and variable thickness. Alternatively, the two electrodes 30 could be of non equal length so that short bars 25 could be more or less than half the length of long bars 24. In still another embodiment additional drop streams and additional electrodes 30 could be employed to produce segmented bars for higher level codes.

Referring to FIG. 3, there is illustrated in enlarged form a typical printed bar such as one of the long bars 24. The bar as illustrated in FIG. 3 comprises thirty printed dots; each dot being the roughly circular mark made upon print receiving medium 26 by impact of one of drops 16. These dots as shown are formatted into an overlapping 10 X 3 matrix. The three-dot-width feature is produced by adjusting the drop stimulation frequency such that each filament 14 generates three drops 16 during the time period of one clock pulse. Mutual overlapping of the three dot columns is achieved by adjusting the movement speed of print receiving medium 26. For a typical bar as illustrated in FIG. 3, the dot diameter may be about 12 mils and the drop stimulation frequency may be about 50 kiloHertz. Thus to produce overlap in the illustrated amount the speed of print receiving medium 26 will be about 335 inches per second. Slower speeds will produce greater overlap and faster speeds will produce printing voids within the bar.

The bar illustrated in FIG. 3 also has printed dot overlap in the direction of the bar axis. Thus five overlapping dots 27a define the length of a short bar, and these five dots in combination with five overlapping dots 27b define the length of the long bar. It may be noted that all of dots 27a and 27b are printed simultaneously or nearly so. Overlapping dots 28a and overlapping dots 29a also define the length of a short bar and overlapping dots 28b and 29b cooperate therewith for also defining the long bar length. The relationship between a short bar and a long bar is emphasized in FIG. 3 by shading of the short bar defining dots 27a, 28a and 29a.

Illustrated in phantom lines within dots 27a and 27b are a series of small circular areas 37 which correspond in spacing and area to orifices 13. Areas 37 also correspond in spacing and area to the cross sectional area and spacing of filaments 14. For the scale of FIG. 3, areas 37 represent circles of about l.8 mil diameter with a center to center spacing of 10 mils. Thus it will be observed that the printed dot made by each depositing drop 16 is considerably larger than the orifice from whence the drop came. This is an operational requirement which necessarily results from the use of an orifice plate 12 having a single row of spaced orifices 13.

Within limits, adjustments to achieve printed dot overlap in the bar axis direction may be effected by adjusting the drop stimulation frequency and/or the pressure applied to the ink 10in manifold 11. More particularly, the dot diameter increases either with a decreasing stimulation frequency or an increasing ink supply pressure. For any given printer configuration the appropriate fluid pressure and stimulation frequency are most conveniently established by a simple trial and error procedure. However, for a starting point the stimulation frequency should be somewhere near the natural drop formation frequency which is well known to have a wave length of about 4.5 times the crosssectional diameter of the fluid filament. For efficient stimulation it is desirable not to deviate any very large amount from this frequency.

If it is desired to avoid trial and error procedures, a set of operating conditions may be reasonably well established by analytical techniques. In this regard it has been found that the diameter of a printed dot is very nearly an exponential function of the momentum of the dot producing drop. For instance in a series of experiments using various sized orifices and a pound basis weight duplicator paper for a print receiving medium, it was found that the dot diameter in mils was very nearly equal to l 1.8 times an exponential factor found by raising the drop momentum to the 0.47 power. For purposes of this calculation the drop momentum is expressed in micro dyne-sec.

To calculate the size of a printed dot expected to be produced by a printer having a known fluid supply pressure, a known stimulation frequency, and orifices of known geometry and known discharge characteristics, the first step is to apply classical techniques for computing the mass flow rate through an orifice. For this step reference may be made for instance to Hydrodynamics by I-l. Lamb, Dover Publications, 1945. Knowing the mass flow rate, the stream diameter, and the stimulation frequency, it becomes a simple matter to determine the drop mass and velocity. See for instance R. G. Sweets report entitled High-Frequency Oscillography with Electrostatically Deflected Ink Jets, Standored Electronics Lab SEL-64-004. The drop mass and velocity determine the drop momentum which may be related to printed dot diameter as above discussed.

Upon carrying through the above analysis it may be verified that increases in fluid pressure increase the printed dot diameter by increasing both the mass and the size of the drop. For a given fluid pressure a decreasing stimulation frequency increases printed dot diameter by increasing the mass of a drop. In most cases, however, the trial and error technique is superior to analysis because it is difficult to estimate the discharge characterics of orifices 13, and these characterics profoundly effect the mass flow rate of the ink therethrough. For the particular embodiment described above, it has been found that satisfactory overlapping 12 mil dots may be produced by operating 1.8 mil orifices at a pressure of 9 psi and stimulating the streams at 50 kilol-Iertz. These numbers are accurate only for one very specific type of orifice plate though and may be somewhat in error if applied to a bar code printer using other slightly different orifice plates.

More particular construction details of the apparatus of FIG. 1 are illustrated in FIGS. 4 and 5. Thus an ink inlet tube 38 and a vacuum outlet tube 39 are provided with access to the interior of manifold 11. A pair of mounting bars 40 are fastened to a manifold 11 for mounting the print head on a holding fixture (not shown). A support plate 42 secures orifice plate 12 in position between manifold 11 and a gasket 41.

Charge strips 17 are mounted in a deep recess in the underside of support plate 42 as shown in FIG. 5. A pair of brackets 44 are seated in a pair of shallow recesses in the underside of support plate 42 for securing charge strips 17 in place. Catcher 21 and deflection electrode assembly 19 also fit into a shallow recess on the underside of support plate 42. Deflection electrode assembly'19 and catcher 21 are held in place by a pair of brackets 43.

In the embodiment described above, printing of bar codes is accomplished by catching groups of charged drops forming a portion of a falling drop curtain and permitting uncharged drops to strike a print receiving medium. The invention could also be practiced by catching the uncharged drops and printing with groups of charged drops. The choice is merely a matter of catcher placement. As another embodiment, the our tain of falling drops could be coded into drop groups of two discrete non-zero charge levels. In an arrangement of this type the drop charge coding would produce trajectory coding by deflecting the groups of drops by different amounts. Printing again would be effected by catching one drop group and permitting the other drop group to impact on the print receiving medium.

While the methods and forms of apparatus herein described constitute preferred embodiments of the invention, it is to be understood that the invention is not limited to these precise methods and forms of apparatus, and that changes may be made therein without departing from the scope of the invention.

What is claimed is:

1. Bar code printing apparatus comprising:

1. an orifice plate provided with a series of uniformly sized orifices regularly spaced along a straight line,

2. means for supplying a printing fluid under jet forming pressure to all of said orifices, thereby creating a line of uniformly sized and regularly spaced jets at the exit side of said orifice plate,

3. means for stimulating said jets and causing them to break-up into streams of uniformly sized and regularly spaced drops,

4. a common extended charging electrode responsive to a binary charging signal for charging to a common level those drops in at least two adjacent streams which are generated during one state of said binary charging signal,

5. means for generating an electrostatic deflection field for deflecting drops which are charged as aforesaid and thereby arranging the associated streams of drops into drop groups alternatively having deflected and non-deflected trajectory characterizations in correspondence with the changing binary state of said charging signal,

6. means for transporting a drop receiving medium transversely across the paths of said drop groups, and

7. means for catching drop groups of one of said trajectory characterizations whereby the drop groups of the other of said trajectory characterizations impact on said drop receiving member to print a series of bars corresponding to said charging signal.

2. Apparatus according to claim 1 further comprising a second common extended charging electrode responsive to a second binary charging signal for charging to a common level those drops in at least two other adjacent streams which are generated during one state of said second binary charging signal, said electrostatic deflection field generating means, said transporting means, and said catching means acting as aforesaid on the drops in said at least two other adjacent streams whereby a second series of bars is printed corresponding to said second charging signal.

3. Apparatus according to claim 2 the first aforesaid electrode commonly charging drops within all of said streams propagating from jets situated on one side of an interior point within said straight line and said second common charging electrode commonly charging drops within all of said streams propagating from jets situated on the other side of said interior point whereby said series of bars and said second series of bars are printed in side-by-side relationship.

4. Apparatus according to claim 3 said catching means being arranged for catching of drop groups having a deflected trajectory characterization whereby said pair of bar codes are printed by uncharged and undeflected drops.

5. Apparatus according to claim 4 said second charging electrode being responsive to the first aforesaid binary charging signal and to said second binary charging signal whereby said printing apparatus prints a bar code consisting of long and short bars.

6. Apparatus according to claim 5 said interior point being a midpoint whereby said short bars are half the length of the long bars.

7. Bar code printing apparatus comprising:

1. an orifice plate provided with a series of orifices spaced along a line,

2. means for supplying a printing fluid under jet forming pressure to all of said orifices, thereby creating a line of spaced jets at the exit side of said orifice plate,

3. means for stimulating said jets and causing them to break up into streams of regularly spaced drops having sufficient mass and velocity for printing dots with side-by-side overlap,

4. a common extended first charging electrode responsive to a first charging signal for charging to a common level all of said drops generated during the period of said first charging signal and propagating from jets situated on one side of an interior point within said line,

5. a common extended second charging electrode responsive to a second charging signal for charging to a common level all of said drops generated during the period of said second charging signal and propagating from jets situated on the other side of said interior point within said line,

6. means for generating an electrostatic field for deflecting all drops charged as aforesaid in response to said first and second charging signals,

7. means for catching all drops deflected as aforesaid,

and

8. means for transporting a print receiving member at a predetermined speed across the paths of the non-caught drops for deposition of said drops thereon; the speed of movement of said print receiving member being such that printed dots created by successively generated drops overlap in the direction of said movement.

8. Apparatus according to claim 7, said interior point being a midpoint within said line.

9. In a bar code printing system comprising means for generating first and second binary information signals and means for printing a bar code corresponding therewith, the improvement wherein said printing means comprises: r

1. an orifice plate provided with a series of orifices spaced along a line,

2. means for supplying a printing fluid under jet forming pressure to all of said orifices, thereby creating a line of spaced jets at the exit side of said orifice plate,

3. means for stimulating said jets and causing them to break up into streams of uniformly sized drops,

4. a first extended electrode for common charging of drops propagating from jets situated on one side of an interior point within said line,

5. a second extended electrode for common charging of drops propagating from jets situated on the other side of said interior point,

6. means for applying a charging potential to said first electrode in correspondence with occurrences of one binary state of said first information signal,

7. means for applying a charging potential to said second electrode in correspondence with occurrences of one binary state of said second information signal,

8. electrical field generating means for deflecting drops charged by said first and second electrodes in response to the application of charging potentials thereto,

9. means for catching the drops deflected as aforesaid, and

10. means for transporting a print receiving member across the paths of the non-caught drops.

10. The improvement of claim 9 said electrodes both comprising flat strips of conductive material.

11. The improvement of claim 9, the size and spacing of said orifices, the operating pressure of said fluid supply means and the operating frequency of said stimulating means all being such that said drops have sufficient mass and velocity for printing a first series of overlapping dots defining a series of bars corresponding to said first information signal and a second series of overlapping dots defining a series of bars corresponding to said second information signal.

12. Apparatus according to claim 1 said electrode comprising a flat strip of conductive material.

13. Apparatus according to claim 1 said drops having sufficient mass and velocity for printing dots with sideby-side overlap. 

1. Bar code printing apparatus comprising:
 1. an orifice plate provided with a series of uniformly sized orifices regularly spaced along a straight line,
 2. means for supplying a printing fluid under jet forming pressure to all of said orifices, thereby creating a line of uniformly sized and regUlarly spaced jets at the exit side of said orifice plate,
 3. means for stimulating said jets and causing them to break-up into streams of uniformly sized and regularly spaced drops,
 4. a common extended charging electrode responsive to a binary charging signal for charging to a common level those drops in at least two adjacent streams which are generated during one state of said binary charging signal,
 5. means for generating an electrostatic deflection field for deflecting drops which are charged as aforesaid and thereby arranging the associated streams of drops into drop groups alternatively having deflected and non-deflected trajectory characterizations in correspondence with the changing binary state of said charging signal,
 6. means for transporting a drop receiving medium transversely across the paths of said drop groups, and
 7. means for catching drop groups of one of said trajectory characterizations whereby the drop groups of the other of said trajectory characterizations impact on said drop receiving member to print a series of bars corresponding to said charging signal.
 2. means for supplying a printing fluid under jet forming pressure to all of said orifices, thereby creating a line of uniformly sized and regUlarly spaced jets at the exit side of said orifice plate,
 2. means for supplying a printing fluid under jet forming pressure to all of said orifices, thereby creating a line of spaced jets at the exit side of said orifice plate,
 2. means for supplying a printing fluid under jet forming pressure to all of said orifices, thereby creating a line of spaced jets at the exit side of said orifice plate,
 2. Apparatus according to claim 1 further comprising a second common extended charging electrode responsive to a second binary charging signal for charging to a common level those drops in at least two other adjacent streams which are generated during one state of said second binary charging signal, said electrostatic deflection field generating means, said transporting means, and said catching means acting as aforesaid on the drops in said at least two other adjacent streams whereby a second series of bars is printed corresponding to said second charging signal.
 3. Apparatus according to claim 2 the first aforesaid electrode commonly charging drops within all of said streams propagating from jets situated on one side of an interior point within said straight line and said second common charging electrode commonly charging drops within all of said streams propagating from jets situated on the other side of said interior point whereby said series of bars and said second series of bars are printed in side-by-side relationship.
 3. means for stimulating said jets and causing them to break up into streams of regularly spaced drops having sufficient mass and velocity for printing dots with side-by-side overlap,
 3. means for stimulating said jets and causing them to break-up into streams of uniformly sized and regularly spaced drops,
 3. means for stimulating said jets and causing them to break up into streams of uniformly sized drops,
 4. a first extended electrode for common charging of drops propagating from jets situated on one side of an interior point within said line,
 4. a common extended charging electrode responsive to a binary charging signal for charging to a common level those drops in at least two adjacent streams which are generated during one state of said binary charging signal,
 4. Apparatus according to claim 3 said catching means being arranged for catching of drop groups having a deflected trajectory characterization whereby said pair of bar codes are printed by uncharged and undeflected drops.
 4. a common extended first charging electrode responsive to a first charging signal for charging to a common level all of said drops generated during the period of said first charging signal and propagating from jets situated on one side of an interior point within said line,
 5. a common extended second charging electrode responsive to a second charging signal for charging to a common level all of said drops generated during the period of said second charging signal and propagating from jets situated on the other side of said interior point within said line,
 5. Apparatus according to claim 4 said second charging electrode being responsive to the first aforesaid binary charging signal and to said second binary charging signal whereby said printing apparatus prints a bar code consisting of long and short bars.
 5. means for generating an electrostatic deflection field for deflecting drops which are charged as aforesaid and thereby arranging the associated streams of drops into drop groups alternatively having deflected and non-deflected trajectory characterizations in correspondence with the changing binary state of said charging signal,
 5. a second extended electrode for common charging of drops propagating from jets situated on the other side of said interior point,
 6. means for applying a charging potential to said first electrode in correspondence with occurrences of one binary state of said first information signal,
 6. means for transporting a drop receiving medium transversely across the paths of said drop groups, and
 6. Apparatus according to claim 5 said interior point being a midpoint whereby said short bars are half the length of the long bars.
 6. means for generating an electrostatic field for deflecting all drops charged as aforesaid in response to said first anD second charging signals,
 7. means for catching all drops deflected as aforesaid, and
 7. Bar code printing apparatus comprising:
 7. means for catching drop groups of one of said trajectory characterizations whereby the drop groups of the other of said trajectory characterizations impact on said drop receiving member to print a series of bars corresponding to said charging signal.
 7. means for applying a charging potential to said second electrode in correspondence with occurrences of one binary state of said second information signal,
 8. electrical field generating means for deflecting drops charged by said first and second electrodes in response to the application of charging potentials thereto,
 8. Apparatus according to claim 7, said interior point being a midpoint within said line.
 8. means for transporting a print receiving member at a predetermined speed across the paths of the non-caught drops for deposition of said drops thereon; the speed of movement of said print receiving member being such that printed dots created by successively generated drops overlap in the direction of said movement.
 9. In a bar code printing system comprising means for generating first and second binary information signals and means for printing a bar code corresponding therewith, the improvement wherein said printing means comprises:
 9. means for catching the drops deflected as aforesaid, and
 10. means for transporting a print receiving member across the paths of the non-caught drops.
 10. The improvement of claim 9 said electrodes both comprising flat strips of conductive material.
 11. The improvement of claim 9, the size and spacing of said orifices, the operating pressure of said fluid supply means and the operating frequency of said stimulating means all being such that said drops have sufficient mass and velocity for printing a first series of overlapping dots defining a series of bars corresponding to said first information signal and a second series of overlapping dots defining a series of bars corresponding to said second information signal.
 12. Apparatus according to claim 1 said electrode comprising a flat strip of conductive material.
 13. Apparatus according to claim 1 said drops having sufficient mass and velocity for printing dots with side-by-side overlap. 